During production of oil and gas from a subterranean reservoir, the well stream will normally contain oil, gas, water and some solid particles. In order to separate the various fluids and solids, a dedicated process system for the well stream is constructed. The separation is made in several stages, where the “bulk separation” of the various phases is carried out by gravity forces alone, and the “fine separation” or purification is mainly utilising centrifugal forces and inertial forces together with the gravity force. A major challenge appearing in most separation stages is to remove liquid droplets from a gas stream where the liquid content in the gas is low, typically less than 1 vol % of the total volumetric flow. It is of outmost importance to remove most of this liquid in order to protect downstream equipment such as compressors and additional dewatering equipment. This is done in large separators, horizontally or vertically oriented. In the following, separators dedicated to separate gas/liquid mixtures containing less than said 2 vol % liquid are denoted gas scrubbers. This denotation also comprises contactor towers used for removing vapour from a gas stream, where the lowermost and the uppermost parts of the contactor tower are used to remove liquid droplets from the gas stream.
In gas scrubbers, the separation takes place in several stages. First, the gas enters through an inlet nozzle, which—for vertical oriented scrubbers—may be located approximately at the middle of the scrubber in its vertical direction. At the inlet nozzle a momentum breaker plate, a vane diffuser or any device may be arranged in order to distribute fluids across the separator cross-sectional area. Already here, the largest drops are separated and fall down onto the liquid reservoir in the lower part of the separator.
The gas flows upwards into a calm zone, or deposition zone, where further droplets due to gravity fall down onto the liquid surface below, alternatively first deposit on the separator wall and thereafter drain downwards on the wall.
Close to the top of the separator, the gas is made to pass through droplet separation equipment of known technology. There are mainly three categories of droplet separation equipment; mesh pad, vanepack and axial flow cyclones arranged in parallel. Because of the pressure drop across the droplet separation equipment, the separated liquid is normally drained down to the liquid 4reservoir through a drain pipe, the lower end of which is submerged in the liquid reservoir. It is important that the drain pipe cross-sectional area is sufficient large to allow liquid to coalesce in the drain pipe. If the drain pipe diameter is too small, foam might build up in the drain pipe causing separated liquid not to drain sufficiently, but rather follow the gas.
It is important that the separator inlet device is correctly designed in relation to the separator cross-sectional area in order to remove as much liquid as possible, in order to avoid overloading the demisting equipment. This is particularly important for vertical scrubbers and contactor columns utilised to remove aqueous vapour from a gas stream. Too much liquid fed to the demisting equipment due to poorly designed inlet devices and/or too small scrubber diameters relative to the gas flowrates are the main reasons for malfunction experienced on a large number of scrubber installations. This is due to the fact that vertical scrubbers utilise inlet devices of known technology, where gravity forces alone is used to separate liquid in the scrubber inlet compartment, which sets stringent limits to acceptable gas velocities, above which considerable amounts of liquid is following the gas to the demisting equipment. Inlet cyclones have successfully replaced earlier used inlet devices in modern 2- and 3-phase separators where the liquid content is high, typically above 5 vol %, while inlet devices denoted as vane diffusers still represent state of the art technology in vertical gas scrubbers where the liquid fraction is less than 2 vol %. Lately, cyclone inlet devices are also applied in gas scrubbers. However, some operational problems associated with cyclone inlet devices used in gas scrubbers will be explained in the following.